Abraham used this rhetorical question because he did not believe that this could happen. AT: "Surely a man who is a hundred years old cannot father a child!" (See: [[rc://en/ta/man/translate/figs-rquestion]])
Again Abraham used a rhetorical question because he did not believe this could happen. The phrase "who is ninety years old" tells why Abraham did not believe that Sarah could bear a son. AT: "Sarah is ninety years old. Could she bear a son?" or "Sarah is ninety years old. Surely she could not bear a son!" (See: [[rc://en/ta/man/translate/figs-rquestion]] and [[rc://en/ta/man/translate/figs-distinguish]])
"Please let Ishmael inherit the covenant that you have made with me" or "Perhaps Ishmael could receive your covenant blessing." Abraham suggested something that he believed really could happen.